_for_binlog system variable is enabled, there is no InnoDB
gap locking except for foreign-key constraint checking and duplicate-key
checking. Also, record locks for non-matching rows are released after MariaDB
has evaluated the WHERE condition.If you use READ COMMITTED or enable
innodb_locks_unsafe_for_binlog, you must use row-based binary logging.

REPEATABLE READ
---------------

This is the default isolation level for InnoDB. For consistent reads, there is
an important difference from the READ COMMITTED isolation level: All
consistent reads within the same transaction read the snapshot established by
the first read. This convention means that if you issue several plain
(non-locking) SELECT statements within the same transaction, these SELECT
statements are consistent also with respect to each other. See
http://dev.mysql.com/doc/refman/en/innodb-consistent-read.html.

For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), UPDATE, and
DELETE statements, locking depends on whether the statement uses a unique
index with a unique search condition, or a range-type search condition. For a
unique index with a unique search condition, InnoDB locks only the index
record found, not the gap before it. For other search conditions, InnoDB locks
the index range scanned, using gap locks or next-key (gap plus index-record)
locks to block insertions by other sessions into the gaps covered by the range.

This is the minimum isolation level for non-distributed XA transactions.

SERIALIZABLE
------------

This level is like REPEATABLE READ, but InnoDB implicitly converts all plain
SELECT statements to SELECT ... LOCK IN SHARE MODE if autocommit is disabled.
If autocommit is enabled, the SELECT is its own transaction. It therefore is
known to be read only and can be serialized if performed as a consistent
(non-locking) read and need not block for other transactions. (This means that
to force a plain SELECT to block if other transactions have modified the
selected rows, you should disable autocommit.)

Distributed XA transactions should always use this isolation level.

Access Mode
-----------

The access mode specifies whether the transaction is allowed to write data or
not. By default, transactions are in READ WRITE mode (see the tx_read_only
system variable). READ ONLY mode allows the storage engine to apply
optimizations that cannot be used for transactions which write data. Note that
unlike the global read_only mode, READ_ONLY ADMIN (and SUPER before MariaDB
10.11.0) privilege doesn't allow writes and DDL statements on temporary tables
are not allowed either.

It is not permitted to specify both READ WRITE and READ ONLY in the same
statement.

READ WRITE and READ ONLY can also be specified in the START TRANSACTION
statement, in which case the specified mode is only valid for one transaction.

Examples
--------

SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Attempting to set the isolation level within an existing transaction without
specifying GLOBAL or SESSION.

START TRANSACTION;

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
ERROR 1568 (25001): Transaction characteristics can't be changed while a
transaction is in progress

URL: https://mariadb.com/kb/en/set-transaction/https://mariadb.com/kb/en/set-transaction/